Using regular expression to validate latitude and longitude coordinates then display error

<script type="text/javascript">  

var latitude = document.getElementById(lat).value;
var longitude = document.getElementById(lng).value;

var reg = new RegExp("^-?([1-8]?[1-9]|[1-9]0)\.{1}\d{1,6}");

if( reg.exec(latitude) ) {
 //do nothing
} else {
 //error
}

if( reg.exec(longitude) ) {
 //do nothing
} else {
 //error
}

</script>

Feel free to test your regex here:

^-?([1-8]?[1-9]|[1-9]0)\.{1}\d{1,6}

Debuggex Demo

In this case, the regex would match a number that is negative or positive, either (1 digit excluding '0' and a '0') or (one or two digits, the first one excluding 9, both excluding '0'), followed by a decimal point and up to 6 other digits. If that's what you need, then yes, this would work. If you need a different format, post it in a comment and I'll try to work a proper regex.

Googling "regex" should give you all the info you need.

If you just need numbers between -90 and 90 (or -180 and 180) you can just do this:

if (typeof num === 'number' && num <= 90 && num >= -90)
    //valid
else
    //invalid

If you need symbols like ° (degrees) or compass direction, then regex could be beneficial.

Here is a review of a few formats:

http://www.geomidpoint.com/latlon.html